Revision as of 13:49, 27 October 2016

λ-Integrase

The λ‑integrase, originally derived from the λ‑phage, catalyzes the recombination of the phage genome with the chromosomal genome of its host in combination with several assisting proteins. Therefore, two attachment sites are necessary: one located on the bacterial genome (attB) and the other located on the λ‑genome (attP), which also contains several binding sites for regulatory proteins. Having a Tyrosine in its active-site, the λ-integrase belongs to the tyrosine recombinase family.

Figure 1: Crystal structure of the λ-Integrase in interaction with DNA. Created with VMD (Visual Molecular Dynamics). PDB entry, Biswas, T. et al. 2005

The attachment sites contain homologous recognition sequences, called BOB' region (attB) and COC' region (attP). These regions can be connected by the λ‑integrase and the bacterial integration host factor (IHF) via Holliday junction, forming an intasome, a DNA‑protein‑complex, producing hybrid attachment sites attL and attR.

Figure 2: λ-integrase mediated attP/attB-recombination via Holliday junctions.¹

The attP- and attB-strands are nicked by λ-integrases. The active-site amino-acid of the λ-integrase is its Tyrosine 342. The DNA-strands are cleaved via formation of a high energy 3′-phospho-tyrosine intermediate. After cleavage of the two top-strands a four-way Holliday-junction is formed (Figure 2). Here, two free λ-integrase dimers cut the remaining bottom-strands, also via formation of a 3′-phospho-tyrosine intermediate. After the cleavage of the bottom-strands and a rearrangement of the basepairs, the nicked DNA is religated. The whole attp-plasmid is now integrated into the chromosome of the bacterial host. However sequence of the recombination sites attP and attB are not maintained. The integrated DNA is flanked by the two recombined integration-sites attL and attR. The λ-integrase is used by the λ-phage for integration of the phage's genome in context of the lysogenic cycle. (Figure 3)

Figure 3: The λ-phage integrates its whole genome via attP/attB-recombination.²

The λ-Integrase can be used for specific genomic integration of a variable gene of interest (GOI) via attP/attB recombination. For Integration of a certain GOI it has to be cloned into a vector carrying the λ-attP-site. The cloned plasmid together with the presented Part must be transformed into a λ-negative E. Coli-Strain with attB-site. The presented λ-Integrase is specific for the λ-attP-site used in BBa_K1976000.

Figure 4: SDS-Page after expression of λ-integrase gene. The gel band of λ-integrase is highlighted by the red circle.

References

1. Landy, A. (2015). The λ Integrase Site-specific Recombination Pathway, 1–27.
2. Tong, W., Warren, D., Seah, N. E., Laxmikanthan, G., Van Duyne, G. D., & Landy, A. (2014). Mapping the λ Integrase bridges in the nucleoprotein Holliday junction intermediates of viral integrative and excisive recombination. Proceedings of the National Academy of Sciences of the United States of America, 111(34), 12366–71. http://doi.org/10.1073/pnas.1413007111